This invention relates to underwater breathing apparatus of the type wherein breathing gas is recirculated through a canister containing a carbon dioxide absorbent, and more particularly to an improved carbon dioxide absorbent canister that provides temperature and flow control of the breathing gas.
One form of diving equipment suitable for complete life support of saturation divers operating for extended work periods in very cold and deep water is of the semi-closed, mixed gas type that receives make-up gas and hot water for a heated diving suit through an umbilical by which the diver is tethered to a support facility such as an underwater vehicle.
During normal semi-closed circuit operation, breathing gas from the umbilical passes through an absolute pressure regulator into an inhalation bag, and then through a hose connection to an oronasal face mask. Exhaled gas passes from the mask via a hose to an exhalation bag. From the exhalation bag, most of the exhaled gas flows through a carbon dioxide removal canister to the inhalation bag where it is enriched in oxygen by being mixed with the incoming make-up gas from the umbilical, and subsequently rebreathed. A portion of the exhaled gas does not flow through the canister but is exhausted through an exhaust valve to the water to allow for the make-up gas. In this system, the circulation of the gases is effected almost entirely by the pulmonary efforts of the diver himself. The flow rate through the carbon dioxide scrubber or absorbent canister is usually on the order of one cubic foot per minute, or less, with sinusoidal breathing fluctuations being substantially damped out by the breathing bags. Thus, the gas is subjected to a relatively long and constant residence time in the canister as compared to systems using pumping means such as an aspirator to promote the gas circulation. In the latter systems the gas flow rate through a scrubber canister will generally be on the order of ten to twenty cubic feet per minute.
The latter, flow assisted systems, wherein the high flow rate and short residence times have caused a drying effect on the absorbent material that adversely affects the desired carbon dioxide removing, exothermic reaction, have benefited from the improved carbon dioxide absorbent canister described in U.S. Pat. No. 4,193,966, assigned to the assignee, hereof. That canister has a condensate control feature that avoids the drying action and materially lengthens the effective life of a canister charge in the high flow rate system by extracting heat from the moisture laden incoming breathing gas so as to drop sufficient moisture in the absorbent bed to maintain the reaction.
In cold water operations it has been found that the systems relying principally upon pulmonary function to circulate the gas and having a relatively long residence time of the gas in the scrubber canister are not subject to the same absorbent drying problem, but rather to such severe loss of heat to the cold environment that the operation and effective life of the absorbent charge in the canister is much shorter than would be expected from the calculated life expectancy of the charge. Examinations of the used canister charge have usually shown that only a small portion was chemically spent and efforts have been made to counter this by providing insulation or a heated water jacket around the canister and by adding lateral mechanical baffles to force increased contact area between gas and absorbent and avoid "channeling" of the gas along the wall surfaces of the canister, another presumed cause of short effective scrubbing duration, all with dissappointing results.